: I wouldn't call those euphemisms; rather the opposite! But the names of other animals are just as commonplace: bitch, foxy, wolf whistle, dogging, and so on. (VSD)

And then of course, there are 'the birds and the bees'.

A thought does occur to me: many (most?) people will never have seen a wild mammal mating (come to that, outside the farming community, most won't have seen a non-wild mammal mating) but everyone has seen pigeons, sparrows and so on thus engaged. Might this have given rise to a tradition that birds are particularly active in this area?

Though I believe the fact that a number of our cruder words for sexual activities and bodily parts are also the names of birds is more coincidence than anything else: for example, I believe 'tit' as in the bird species is not related to 'tit' as in breast.